In the fabrication of complementary metal oxide semiconductor (CMOS) gate stacks involving high dielectric constant (i.e., k greater than about 4.0, more typically greater than 7.0) dielectrics, ultra-thin (on of the order of less than 1 nm) metal nitride capping layers have been shown to be of considerable utility. In order to use the metal nitride capping layers in manufacturing, it is necessary to devise a method that allows the deposition of such capping layers. Because of the thinness of such capping layers, and their proximity to a particularly delicate part of the device, e.g., the gate insulator, the requirements and constraints on the requisite deposition process are somewhat different than are normally encountered in the deposition of thin films for microelectronic applications.
Because of the thinness of the capping layers, the deposition rate is typically not an important issue. However, for the same reason, uniformity is of particular importance. The proximity to the gate insulator makes the impurity concentration in the films (e.g., C impurities) of particular concern from a chemical point of view, over and above their effect upon resistivity. In addition, it is generally desirable to perform the deposition at as low a temperature (typically less than 400° C.) as possible, in order to mitigate undesirable diffusion and/or solid-state reactions within the substrate.
Because of these requirements, all of the prior art methods commonly used to deposit binary compound films such as metal nitrides may have significant drawbacks. For example, sputter deposition methods are generally undesirable in this application because of the danger of physical damage to the underlying gate insulator. Chemical vapor deposition (CVD) however appears to be a promising method. In CVD, numerous organometallic precursors (such as carbonyls, alkyls, etc) with reasonable vapor pressure exist for a large number of candidate metals. The problem to be faced with conventional CVD methods is in performing the deposition at sufficiently low temperature (less than about 400° C.).
Even if the metal containing precursor can be thermally decomposed cleanly at sufficiently low temperatures, the most common molecular source of nitrogen, NH3, may require excessively high substrate temperatures to effect the transformation into a metal nitride.
One obvious way around the aforementioned problem of high substrate temperature is to use a plasma assisted (or enhanced) CVD process. In a typical plasma assisted CVD process, the organometallic precursor and the nitrogen source would be constituents of a plasma discharge to which the substrate would be exposed. Unfortunately, such an approach would generally run afoul of the requirement for film purity since a plasma is simply too reactive towards the organic ligands of the metal precursor. In addition to metal and reactive nitrogen species, reactive ligand fragments would also be incident upon the substrate, leading to undesirable carbon incorporation.
A more sophisticated deposition method which frequently makes use of plasma-excited species is atomic layer deposition (ALD). Atomic layer deposition of metal nitrides is a two-step process in which the metal precursor is first chemisorbed to saturation coverage on the substrate, the ligands associated with the metal precursor are then “reacted away” and thereafter nitride film is formed by the exposure to reactive nitrogen atoms from a plasma source. Cycles of these two steps are successively performed until the desired film thickness is obtained.
At first glance, the ALD method seems exceedingly promising. In the ideal case described above, uniformity is virtually guaranteed by the chemical nature of the ALD process. However, ALD does impose the additional requirement of finding a precursor that chemisorbs to saturation, both on the initial substrate and upon the growing film, which typically are rather different chemical entities. More importantly, the ALD approach is also potentially subject to purity concerns. That is, metal precursor molecules typically chemisorb to saturation because they cannot shed all of their ligands. Since these ligands are not shed thermally by the precursor on the substrate surface, the ligands are vulnerable to over-aggressive attack by the plasma species resulting in the same purity concerns as in the case of ordinary plasma assisted CVD.
To illustrate how these process requirements affect the deposition methodology, a specific case of a metal nitride capping layer deposition is now mentioned. In particular, the deposition of 1-2 nm layers of aluminum nitride (AlN) is now mentioned. In common with many metal nitride systems, the preferred chemical precursors for Al are organometallic species, specifically trialkyl compounds, or amine adducts of alane, although the latter are less readily available. Focusing on the metal alkyl precursors, the difficulties associated with each of the potential deposition schemes outlined above is now described.
The metal alkyl precursors, especially tris(tertbutyl)aluminum, can be employed to deposit reasonable pure aluminum at moderate temperatures, but reaction with ammonia to form a stoichiometric metal nitride is only efficient at temperatures in excess of 400° C., which restricts the potential applications of such a process. In addition, the extremely high reactivity of the alkyl aluminum precursor with the substrate at these temperatures would render thickness and uniformity control difficult.
Plasma deposition is completely ruled out on purity grounds alone. ALD might seem to be a promising approach, but it is also inapplicable. If the substrate is sufficiently hot, on the order of 250° C. or greater, the alkyl aluminum precursor used in an ALD process spontaneously decomposes on the substrate, shedding all of its ligands, to leave behind pure Al which could be nitrided by a plasma generated species. However, this decomposition process is not self-limiting at a coverage of less than or equal to one monolayer. This self-limiting behavior is an absolute precondition for ALD. If one were to drop the substrate temperature to a point where the alkyl aluminum did not completely decompose, it might be possible to achieve self-limiting chemisorption, however if this were exposed to plasma generated species one would have the same purity concerns that are typically associated with ordinary plasma assisted CVD. That is, the organic ligands on the substrate surface would be subject to decomposition by reactive plasma species, which could lead to carbon incorporation into the film.
By process of elimination, one is thus led to the preferred method for the low temperature deposition of ultra-thin metal nitride films, atomic assisted MOCVD. In this process, the substrate temperature can be maintained sufficiently high to ensure complete clean decomposition of the alkyl aluminum precursor on the substrate, while simultaneously exposing the substrate to highly reactive nitrogen atoms generated in an atom source. The nitrogen flux should be in sufficient excess over the alkyl aluminum flux to ensure that a stoichiometric (maximally nitrided) film is produced. Furthermore, it is important that this nitrogen atom flux be directed at the substrate simultaneously with the alkyl aluminum flux.
Metal nitrides are good diffusion barriers. If the organometallic and atom fluxes were applied sequentially buried aluminum atoms might go unreacted, leading to non-stiochiometric films. To avoid the problem of gas phase reaction between the alkyl aluminum species and the nitrogen atoms, which could lead to alkyl ligand fragments impinging on the growth surface and incorporating carbon, the pressure in the reactor is kept quite low during deposition, preferably below 1 mtorr and most preferably below 0.1 mtorr. At these pressures, gas phase chemistry is suppressed to negligible levels.
In most CVD applications, such low pressures would be quite impractical, as they would lead to unacceptably low deposition rates, however since there is an ongoing desire in growing films of less than 1 nm thickness, even a deposition rate of 0.1 nm/min would be acceptable. This low total pressure requirement does however necessitate that the nitrogen atoms be remotely generated with effective differential pumping with respect to the deposition chamber.
The primary challenge in employing the preferred method is to achieve film uniformity and maintain proper stoichiometry across the film. A standard apparatus for achieving uniformity in MOCVD is the “showerhead” apparatus shown in FIG. 1. In FIG. 1, the showerhead/substrate/heater assembly is placed within a vacuum enclosure 7. The gas comprising the organometallic precursor is introduced into the cavity of the showerhead 3 by means of valve 1 and introduction tube 2. From there the precursor gas is sprayed onto the substrate via a series of nozzles 4, typically a few hundred in number and on the order of 1 mm in diameter, onto the substrate 5, which is situated atop the substrate heater 6. A large number of nozzles 4 and a close proximity of the substrate 5 to the showerhead 3 (typically at most 1 cm) are employed to ensure a uniform thickness across the substrate. For this to be achieved, it is also necessary to establish a substantial pressure differential between the showerhead cavity and the interior of the vacuum enclosure 7 during the deposition. This is to ensure that the showerhead cavity becomes uniformly filled with precursor gas so that each nozzle conveys the same amount of precursor gas to the substrate. Naturally with some specific designs there can be some modest deviations from the idea situation just described.
Such a prior art apparatus would do an excellent job of delivering trialkyl aluminum precursors to the substrate in a uniform fashion, but it renders the problem of delivering nitrogen atoms to the growth surface all but insoluble. The nitrogen atoms cannot be introduced through a valve and tube into the showerhead cavity. Nitrogen atoms will react with and/or recombine very efficiently on any surfaces they encounter. Any nitrogen atoms introduced through such as valve 1 would likely experience thousands of encounters with the cavity walls before escaping through the nozzles 4. Thus, few if any reactive atoms would reach the substrate. Furthermore, due to the higher pressure within the showerhead cavity, deleterious gas phase chemical interactions between the atoms and the metal precursor molecules could take place.
One could attempt to bring the nitrogen atoms in from the side, but the approximate 15:1 ratio of the substrate radius to the showerhead spacing would also lead to depletion problems. In addition, the loss of cylindrical symmetry could lead to azimuthal stoichiometry variations, which could only be overcome, if at all, by the use of multiple costly atom sources.
The design challenge, therefore, for this process and for atomic beam assisted MOCVD generally, is to produce an applicator system which efficiently transports both the organometallic metal precursor and the atomic beam to the substrate, in such a fashion that uniformly thick films across a substrate can be grown. The requirements for uniform flux arriving at the substrate surface are quite different for the atomic beam and the MOCVD precursor. Since the atomic beam and the nitrogen atoms constitute a reagent in excess, it is not necessary to have uniform flux across the surface. It is sufficient simply to ensure that it is everywhere in excess, as beyond that point the nitrogen atoms will merely recombine on the surface to yield nonreactive nitrogen molecules.
The MOCVD precursor flux must be extremely uniform across the entire substrate, as it is this reagent which determines the film thickness. A 1σ non-uniformity of better than 10% is virtually always required, and better than 5% is preferred.
In view of the above, there is a continued need to provide a new and improved atomic beam assisted metal organic chemical vapor deposition process which is capable of providing highly pure ultra-thin films in which the thickness variation of the as deposited film across the deposited surface is less than 1 Å.